Exhaust gases of cars and ships with internal combustion engines contain nitrogen oxide (NOx) such as NO, NO2, N2O, etc., which is a cause of photochemical smog or acid rain. In order to reduce nitrogen oxide emissions, typically useful is a filter for purifying exhaust gases by spraying a reductant such as ammonia or urea onto exhaust gases so that the exhaust gases are converted into harmless N2 and H2O via an active catalytic reaction.
NOx emissions are being strictly regulated by laws. Particularly, International Maritime Organization (IMO) has strengthened regulations for NOx emissions from ship engines so that the NOx emissions will be reduced from 14.4 g to 3.4 g per h from 2016 in order to prevent marine pollution.
Generally, exhaust gases of car engines contain less than 10 ppm of sulfur and have a temperature at 250˜450° C., whereas exhaust gases of ship engines contain 500 ppm or more of sulfur and 10% or more of water and have a temperature of 250˜350° C. A filter for purifying exhaust gases for use in ship engines is required to have resistance to water and sulfur and to be efficiently active at a temperature of 300° C. or less, compared to a filter for purifying exhaust gases for use in cars.
Conventional NOx reduction techniques for use in ships and plants include an extruded catalytic filter manufactured by extruding a catalyst material such as vanadia (V2O5), but such an extruded catalytic filter has a very large volume due to too low cell density and does not satisfy Tier III corresponding to regulations for ships because of drastically lowered catalytic activity at low temperature.
The following [Table 1] shows the results of test of NOx reduction efficiency of the conventional extruded catalytic filter composed of vanadia (V2O5) and PILC (Pillared Interlayer Clay) in the temperature range of 250˜300° C., in which the test conditions are N2 balance, NO 1000 ppm, NH3 1000 ppm, O2 11%, Water 10%, CO 100 ppm, CO2 5%, SO2 500 ppm, THC 100 ppm and SV (Space velocity) 10000 h−1.
TABLE 1Temp. (° C.)250255260265270275280285290295300DeNox (%)6768697274757677798181
As is apparent from [Table 1], it is conventionally impossible to achieve NOx reduction efficiency of 85% or more at 300° C. or less. Hence, there is an urgent need to develop active catalyst techniques adapted for preventing environmental pollution and marine pollution and for strengthening regulations thereof.